But Better Markets' challenge mostly turns on a specific part of the settlement, which included a $2 billion civil money penalty for alleged violations of the Financial Institutions Reform, Recovery and Enforcement Act. The group claims that FIRREA specifically requires Attorney General Eric Holder to submit the fine for judicial review.

"DOJ ignored the explicit provisions in FIRREA that requires a court to assess any civil money penalty sought pursuant to that statute," says the complaint, which was filed Feb. 11 in the U.S. District Court for the District of Columbia.

Dennis Kelleher, the president and chief executive of Better Markets, is also raising a constitutional claim that the Justice Department can't act as an investigator, prosecutor, judge and jury in resolving issues involving the nationís largest bank.

He hopes to use that argument to establish legal standing to bring the case.

"We will establish standing of concrete harm," Kelleher said in an interview.

Some observers said Kelleher's tactic could work, noting that Kelleher has experience as a partner with Skadden, Arps, Slate, Meagher & Flom, specializing in securities and financial markets cases.

"They would not have filed their complaint unless they had researched it and believed it had some change of surviving a motion to dismiss," said Cornelius Hurley, the director of Boston University's Center for Finance, Law and Policy. "They would not spend that amount of time and money to do this if it was just a frivolous venture."

But others are more skeptical.

Ron Glancz, a partner at Venable LLP, said Better Markets will have a hard time qualifying for standing in court.

"I'm not aware of a settlement that can be somehow overturned by a plaintiff that is not a party of the complaint," Glancz said. "If they were suing on behalf of homeowners, that is one thing. But to sue just because they represent financial markets raises serious questions of standing, of ripeness and other defenses that the government will assert.

"This is going to be an uphill battle and very difficult to overturn," he said.

Glancz said that the Justice Department generally does not submit settlements for judicial review unless required by law, such as in fair lending and anti-trust cases.

He added that FIRREA did not appear to prohibit voluntary settlements.

"I donít think it is exclusive. I don't think it prevents a voluntary settlement," said Glancz, a former assistant director of the Justice Department's civil division.

He noted the Justice Department got a "voluntary penalty without having to sue. That is the whole purpose of a settlementóto avoid the expense, the time and uncertainty of going to court."